What is the primary requirement of a Reduced Takeoff Runway Length Procedure (RTRL)?

The primary requirement of a Reduced Takeoff Runway Length Procedure (RTRL) is that the aircraft must lift off at or before a specified distance from the Departure End of Runway (DER). This procedure is designed to enable aircraft to safely take off from shorter runways by establishing criteria that ensure sufficient runway length is available for safe liftoff, given the aircraft's performance characteristics and the operating conditions.

This requirement is essential for ensuring that the aircraft can achieve the necessary speed and altitude as required for takeoff while adhering to safety protocols. The specified distance from the DER is crucial, particularly in scenarios involving obstacles or specific terrain considerations that may impact the aircraft's flight path immediately after takeoff.

The other options touch on important aspects of flight safety and operational procedures, such as maintaining a minimum climb gradient, taking off under favorable weather conditions, and considering runway thresholds, but they do not serve as the primary requirement for an RTRL procedure. The focus of the RTRL is specifically on the lifting off at the specified distance from the DER.